1. Field of the Invention
This invention relates to a method and apparatus for sensing the condition of a gas and more particularly to a method and apparatus for continuously withdrawing a portion of the gas from the main gas stream and sensing the condition of the withdrawn gas.
2. Description of the Prior Art
In various industrial applications .Iadd.as in flue gas analysis .Iaddend.it is necessary to continuously sense the condition of a flowing gas. The conventional methods for sensing the condition of the gas include inserting a probe or a portion of the sensing element into the main stream of flowing gas. Problems are encountered with these conventional methods when the flowing gas stream is dirty or wet or when the gas flow patterns are such that erratic measurements result. The probes or portions of the sensing elements frequently become plugged or contaminated by the wet or dirty gaseous products and provide erratic results. When the sensing apparatus depends on a measurement at a calibrated temperature or a continuous flow rate of the gas, a relatively constant predictable gas flow rate around the sensing element is essential for accurate measurement. An example of such sensing apparatus is .[.a gas.]. .Iadd.an oxygen .Iaddend.analyzer that includes a heated ceramic oxide electrolyte electrochemical cell that measures oxygen partial pressure. Excessive flow rates of the gas across the sensing element tends to cool the electrochemical cell so that it is below its calibration temperature and erratic results are obtained. Also, where the flow rate of the gas is slow or static or where the gas is diluted with air, because of reverse flow, erratic results are also obtained. There is a need for a method and apparatus to continuously sense the condition of a gas by providing a small and predictable continuous flow of a portion of the gas over the outside of the sensing element while the major portion of the gas flows through the main gas passageway in an uncontrolled manner.